This invention relates to gas turbine engines, and more particularly to containment arrangements for fan casings of such engines.
Conventionally, the fan blades of a gas turbine engine rotate within an annular layer of abradable material, known as a fan track, within the fan casing. In operation, the fan blades cut a path into this abradable layer, minimising leakage around the blade tips.
The fan casing incorporates a containment system, designed to contain any released blades or debris if a fan blade should fail for any reason. The strength and compliance of the fan casing must be precisely calculated to absorb the energy of the resulting debris. It is therefore essential that the fan track should not interrupt the blade trajectory in a blade-off event, and therefore the fan track must be relatively weak so that any released blade or blade fragment can pass through it essentially unimpeded to the containment system.
Rearward of the fan track, there is conventionally provided an annular ice impact panel. This is typically a glass-reinforced plastic (GRP) moulding, or a tray or panel of some other material. It may also be wrapped with GRP to increase its impact strength. Ice that forms on the fan blades is acted on both by centrifugal and by airflow forces, which respectively cause it to move outwards and rearwards before being shed from the blade.
The geometry of a conventional fan blade is such that the ice is shed from the trailing edge of the blade, and it will strike the ice impact panel rearward of the fan track. The ice will bounce off, or be deflected by, the ice impact panel without damaging the panel.
Swept fan blades have a greater chord length at their central portion than conventional fan blades. Swept fan blades are increasingly favoured in the gas turbine industry as they offer significant advantages in efficiency over conventional blades. Because of their greater chordal length, ice that forms on such a blade, although it follows the same rearward and outward path as on a conventional blade, may reach the radially outer tip of the blade before it reaches the trailing edge. It will therefore be shed from the blade tip and strike the fan track.
However, a conventional fan track is not strong enough to tolerate ice impact, and so conventional arrangements are not suitable for use with swept fan blades. It is not possible simply to strengthen the fan track to accommodate ice impact, because this would disrupt the blade trajectory during a blade-off event, and compromise the operation of the fan casing containment system.
The gas turbine industry has also favoured the development of lighter fan blades in recent years; such blades are typically either of hollow metal or of composite construction. This development has given rise to another problem. Because the blade is lighter, and therefore its resistance to deformation is lower, it is even more difficult to devise a casing arrangement that will resist the passage of ice and yet not interfere with the trajectory of a released fan blade. Furthermore, lightweight swept blades tend to break up, on impact with a fan casing, in a different way from conventional blades, and conventional casing designs are not designed to accommodate this.
In summary, the developments in the gas turbine industry towards, on the one hand, swept fan blades, and on the other, lighter fan blades, have made it increasingly difficult to design a fan casing and containment arrangement that can deliver the three functions required of such an arrangement—namely an abradable fan track, resistance to shed ice and containment of blades or blade fragments.